Will aeroplanes ever be powered by hydrogen? By Jeremy Thomson, BALPA Flight Safety Rep he history of aviation has always gone hand-in-hand with the exploitation of fossil fuels. Even the Wright brothers first tentative hops into the air in 1903 were made possible only by their carburetted gasoline engine technology that had existed for just a few years. Wind back to 2018 and we see aviation enjoying unprecedented growth. Decarbonisation if it applied to aviation at all was thought likely to be limited to modest offsetting schemes. The idea of taxing jet fuel or carbon emissions, along with switching to alternative fuels and technologies, was largely consigned to the too difficult bucket. What a difference a few years make. Thanks in part to COVID-induced navel gazing, there has been a tectonic shift in the aviation sectors environmental ambitions. ICAOs CORSIA (carbon offsetting and reduction scheme for international aviation, www.icao.int/environmental-protection/CORSIA) programme originally a rather unambitious carbon-trading system is moving to tighten its CO2 targets and mandate increased use of sustainable fuels globally. The EUs Green Deal has just lowered its target to reduce overall CO2 emissions by 60 per cent by 2030 and makes it clear that the aviation sector must play its part. The UK government has convened the Jet Zero council. Airlines, too, are on board; BAs owner, IAG, has targets of 10 per cent CO2 reduction by 2025, 20 per cent by 2030 and 100 per cent by 2050. Coincidentally, the latter figure is shared by the Japanese government, while China is aiming for 2060. So, the debate now is not whether to decarbonise, but how. There are several alternatives to oil-based fuels (see box 1), but right now, the biggest buzz is around hydrogen and its ability not just to clean up aviation, but also, potentially, to reform the worlds entire energy network. Why hydrogen? Hydrogen fuel is easy to produce (see box, overleaf), three times more energetic than jet fuel by mass, and reasonably easy to distribute in gaseous form. It is clean burning and can power lightly modified jet-turbine engines while producing zero CO2, zero sulphur and virtually no particulates. It does produce large quantities of water vapour and some NOx both greenhouse gases but the overall warming effect is still a small fraction of conventional fuels. Burning is not the only way to harness hydrogens power, it can also be used in fuel cells. These combine hydrogen and oxygen to generate electricity directly, without combustion, with water being the only by-product. Onboard fuel cells can be used to supply the aircrafts electrical loads, assist the turbine engines in hybrid power systems or, for smaller aircraft, to power propellers directly via electric motors. But there is a major challenge to be overcome safely storing the hydrogen on the aircraft. Gaseous hydrogen is extremely low density, so the only practical option is to liquefy it. This involves cooling it to around 20 degrees above absolute zero, after which it must be heavily insulated or maintained at pressures up to 10,000psi to prevent it boiling away. Even after all this engineering effort, the liquid hydrogen has a density of just 70g per litre (jet fuel is around 800g per litre), necessitating some very large fuel tanks. READ MORE HOW TO Producing hydrogen is easy. All you need is a tank of water, two electrodes and a power source. Switch it on, and the water molecules split into bubbles of hydrogen at one electrode and oxygen at the other. This is electrolysis, and can produce hydrogen on an industrial scale. Using renewable electricity, it is carbon-neutral. Unfortunately, only a fraction of the worlds annual supply of hydrogen (70m tonnes) is produced this way. The rest comes from chemically reforming fossil fuels and produces huge quantities of CO2. Capture and storage of this waste CO2 is a possible interim option to reduce the environmental impact while the renewable energy sector scales up. The relatively conventional versions of the Airbus ZEROe aircraft Getting airborne with hydrogen Airbus is already working on these problems, and has published a bold ambition to have commercial hydrogen-powered aircraft in service by 2035. It has released a suite of ZEROe concept aircraft. The first two designs look relatively conventional a short-haul turbofan and regional turboprop aircraft but both propose to burn liquid hydrogen directly in modified gas turbine engines, and allocate space in the tail to the storage and distribution of the hydrogen fuel. Airbus is also touting a new variant of its eye-catching blended-wing-body concept, arguing that the shape better incorporates the large fuel tanks, and proposing new hybrid electric/hydrogen turbofan engines. At the other end of the scale, the HyFlyer project has American innovator ZeroAvia teaming up with Cranfield University to develop a fleet of 10-20-seat, zero-emission, hydrogen-powered aircraft for local transport. It has already flown a proof-of-concept aircraft: a six seater Piper Malibu. Onboard hydrogen tanks feed a fuel cell, powering the prop via an electric motor. Hoping to achieve a range of 250 miles by the end of this year, its ultimate aim is to provide short-haul services up to 500 miles at half the current operating cost, using regional airports to provide a new network of green, local air travel. To support the aircraft, the consortium is also developing an entire airport ecosystem of hydrogen production, storage and distribution at Cranfield airport. READ MORE Box 1 Alternative fuels and how they compare HOW TO Biofuels Synthetic fuel from waste Electro-fuels (fuels synthesised chemically from CO2 and water) Hydrogen Battery power Advantages Challenges No changes to aircraft or fuel-distribution systems. Can be blended with fossil fuels. Available now. Competes with agriculture. Complex and energy-intensive to produce. Still emits CO2 and other pollutants at altitude. No changes to aircraft or fueldistributions systems. Can be blended with fossil fuels. Available now. Limited scope because of availability of suitable feedstock. Still emits CO2 and other pollutants at altitude. Expensive. Time needed to scale up. No changes to aircraft or fueldistribution systems. Cleaner burning than Jet-A. Requires large amounts of renewable electricity and waste CO2. Still emits CO2 and other pollutants at altitude. Clean burning. Can be used in fuel cells. Requires huge amounts of renewable energy and/or largescale carbon capture technology. Major engineering challenges for tanks and engines. Combustion-free, quiet, pollution-free at point of use. Currently has nowhere near sufficient energy density. Expensive. Charging time might be a problem. The bigger picture Aviation is not the only application of green hydrogen. Trains and shipping would both benefit, and arent short of the necessary tank-storage space. Ground transport could follow once safety issues are addressed. Thinking more widely, green hydrogen can be blended with the domestic natural gas supply and is an essential feedstock for the production of electro-fuels. It even has the potential to reform the National Grid, solving its energy-storage problems and providing an alternative means of energy transmission. In fact, hydrogen could replace carbon in our global energy network. Leading the way is the German government, with its national hydrogen strategy. It sees hydrogen infrastructure as the only way to meet its target of 80-95 per cent CO2 reduction by 2050. Its complex plan integrates production, distribution and consumption of hydrogen, backed up by a quadrupling of its renewable energygeneration capacity. Clearly, such a tectonic shift in infrastructure will take great political will and a gargantuan investment of time and effort. It offers a solution for sustainable transport, power and manufacturing, but only in the medium to long term. So, will many of us be piloting hydrogen-powered aircraft in our careers? I wouldnt bet on it. Most likely, air transport will instead achieve its CO2 reduction goals in the near term via synthetic and electro-fuels. One thing is clear though aviations 117year love affair with fossil fuels has finally turned sour.